This invention relates generally to the field of light emitting diode (xe2x80x9cLEDxe2x80x9d) circuits. More specifically, this invention relates to the field of backlighting of liquid crystal displays (xe2x80x9cLCDsxe2x80x9d) and other devices with a flexible circuit design.
Backlighting for active matrix liquid crystal displays (xe2x80x9cAMLCDxe2x80x9d) typically uses a cold cathode fluorescent lamp (xe2x80x9cCCFLxe2x80x9d) device. CCFL devices tend to have high back lighting efficacies. CCFL devices have numerous drawbacks. For example, CCFL devices may contain Mercury, a highly dangerous substance that has been banned from many AMLCD applications. CCFL devices may have poor efficacy at lower temperatures, which requires additional circuitry such as a heater element or a boost current circuit. CCFL devices may have a non-linear efficacy curve with respect to temperature. CCFL devices may require an inverter to drive the CCFL device. CCFL devices may require complex control schemes, including light sensors and temperature sensors to provide adequate dimming ratios for night time operations. CCFL devices may have a short life expectancy, especially at lower operating temperatures, and may require additional electromagnetic interference (xe2x80x9cEMIxe2x80x9d) shielding and electric filtering.
Alternatives to CCFL devices for back lighting an AMLCD include Xenon-based devices. Xenon-based backlighting circuits do not contain Mercury, have superior low temperature life expectancy and low temperature operational characteristics, and have less phosphor degradation than CCFL devices. While Xenon lamps correct many of the problems of the CCFL lamp technology, the Xenon lamp technology creates many new problems. For example, Xenon lamps tend to be relatively expensive and require complex control circuitry. Xenon lamps have low efficacy. For example, a Xenon lamp with twice the diameter may provide only half the brightness of a mercury based CCFL lamp. Because the efficacy of the Xenon lamp may be less than half of a CCFL lamp, the additional power needed to power a Xenon based circuit creates a problem of power consumption.
Conventional LED drive circuits have been implemented on conventional rigid circuit boards. Side-lighting LEDs can be used with a rigid circuit board, however, side-lighting LEDs are relatively expensive. Side-lighting LEDs are available in a limited height range, which may not be appropriate for all applications, especially lighting a light pipe. An alternative to using side-lighting LEDs is to use multiple rigid circuit boards. For example, xe2x80x9cdaughterxe2x80x9d cards can be connected to xe2x80x9cmotherboardsxe2x80x9d in a perpendicular configuration. Such perpendicular connection require connectors that consume substantial space and may have reliability problems.
This invention provides a flexible circuit board mounted with top-light LEDs and LED control circuits that provides a convenient and space efficient way to backlight an LCD.
In one aspect, a light emitting diode device has a light emitting diode, a light emitting diode drive circuit, and a flexible circuit board. The flexible circuit board has a tab region. The light emitting diode and the light emitting diode drive circuit are mounted on a first side of the flexible circuit board. The tab region is folded such that the light emitting diode emits light toward a center region of the flexible circuit board.
In another aspect, a liquid crystal display device has a light pipe, a light emitting diode, a light emitting diode control circuit, a flexible circuit board, a light extracting surface, a diffuser, a reflective polarizer, and a liquid crystal display. The flexible circuit board has a tab region. The light emitting diode and the light emitting diode control circuit are connected with the tab region of the flexible circuit board such that the light emitting diode emits light into the light pipe. The light extracting surface is located near a first side of the light pipe. The diffuser is located near a second side of the light pipe. The first and second sides are opposite each other. The reflective polarizer is located above the diffuser. The liquid crystal display is located above the reflective polarizer. The light pipe emits light that passes through the diffuser, the reflective polarizer, then the liquid crystal display.
In a further aspect, a light emitting diode has a top lighting light emitting diode, a light emitting diode drive circuit, and a flexible circuit board having a tab region. The tab region of the flexible circuit board is folded at substantially a right angle relative to a center region of the flexible circuit board. The light emitting diode is mounted with a first side of the tab region. The light emitting diode drive circuit is mounted on a second side of the tab region.
Other systems, methods, features, and advantages of the invention will be or will become apparent to one skilled in the art upon examination of the following figures and detailed description. All such additional systems, methods, features, and advantages are intended to be included within this description, within the scope of the invention, and protected by the accompanying claims.